?- current_prolog_flag(F,Version),sub_atom(F,_,_,_,version),current_prolog_flag(max_procedure_arity,A).
A = 255, F = version_data, Version = ciao(1,25,0,commit_info(master,b868c430bd7e2997db8672f2f85c432c3dfc7ab5,'2026-03-05 18:53:14 +0100','1.25-v1.21-1934-gb868c430bd')) ?
So now you have added this flag, even if in your implementation max_arity would be good enough.
I very much doubt that other currently conforming implementations are willing to add that flag, which to them is useless.
Codifying that max_procedure_arity specifies that restriction *in case it is present* seems to be good enough. That means, that other implementations are now restricted in the sense that if they add that flag, it must have the current meaning, so they cannot use that flag name to mean something else.